It gets funner when it's a value that actually is a complex number with an "imaginary" component, but gets represented by NaN because the real value doesn't make sense by itself. In reality, imaginary components are extremely critical for some equations and are totally valid.

I wish I remembered my math well enough to have one off the top of my head, but I don't use it often enough... IIRC, it's even important for magnetics, let alone more complex physics, so it does come up in practice quite often. I think there's even a fundamental part related to derivatives, but yea, don't exercise math enough. Might be misremembering.